Radars are object-detection systems that use radio waves transmitted and received by an antenna to determine the range, angle, and/or velocity of objects. Most often, radars employ array antennas consisting of multiple antenna elements or sensors that are arranged and interconnected to form an array. Antenna arrays may be directional in that they are designed to focus the antennas radiation pattern towards a particular direction. The direction of the radiation pattern is given by the main beam lobe, which is pointed in the direction where the bulk of the radiated power travels. The directivity and gain of an antenna array can be expressed in terms of the antenna's normalized field strength and array factor, which are fundamental principles of antenna array theory and well known in the art.
The efficiency of an antenna array in terms of directivity and gain depends on the design and geometry of the antenna array. Antenna arrays are generally designed for optimum directivity with high angular resolution. However, angular resolution is proportional to the size of the antenna aperture and the number of antenna elements in the array. High angular resolution requires a large aperture with a large number of antenna elements, which increases the cost of the antenna. In addition, the size of the aperture and number of elements is limited by the antenna element spacing due to ambiguities that arise in widely-spaced antenna arrays. These ambiguities are generally a consequence of grating lobes, which refer to a spatial aliasing effect that occurs when radiation pattern side lobes become substantially larger in amplitude, and approach the level of the main lobe. Grating lobes radiate in unintended directions and are identical, or nearly identical, to the main beam lobes.
In most modern radar systems, antennas arrays are phased arrays configured to steer the main lobe of the radiation pattern in a particular direction. A phase shifter connected to each antenna element, or group of elements, is configured to shift the phase of the signals emitted from the antenna elements in order to provide constructive and/or destructive interference, thereby steering the beams in a desired direction while suppressing those in undesired directions. Phased array antennas having a narrow beam width advantageously have high spatial resolution. However, the scan range (i.e., field-of-view) of planar phased arrays are generally limited to 120° (60° left and 60° right) due to gain degradation of the main lobe as it is steered beyond 60° from boresight (i.e., end-fire angles). The gain degradation and diminished angular resolution in the end-fire scan angles is in part due to the occurrence of grating lobes.